1. Field of the Invention
This invention relates to a clamping device which holds an article to be clamped accurately and firmly and which can finely adjust the distance between clamping faces of the clamping device according to the thickness tolerance of the article to be clamped.
2. Description of the Prior Art
In a numerically controlled machine tool such as a machining center, a pallet, on which a work is mounted, is used to improve the efficiency of mounting a work on, and demounting the same from the worktable of the machine tool. In this case, the positioning and clamping accuracies of the pallet on the worktable has a direct influence on the machining accuracy of a workpiece. Therefore, a clamping device, to which a pallet is changeably positioned and clamped, may be secured to the worktable of the machine tool.
Such a clamping device has a vertically movable clamp plate on a clamp base fixed to the worktable of the machine tool. When a pallet is transferred to the clamp plate and placed in a predetermined position, engagement elements formed on the underface of the pallet are clamped between the clamp plate and the clamp base.
When a pallet is clamped, it must be accurately positioned on the clamping device. For this reason, the clamping device is provided with a positioning pin, which is inserted into a recess formed in the pallet by a hydraulic, pneumatic or electromagnetic actuator.
When a pallet is transferred onto the clamp plate, it is necessary to temporarily position the pallet thereon so that the recess of the pallet is located right above the positioning pin. Therefore, there is provided a stop, against which the pallet being transferred to the clamp plate abuts at its forward end to be set in a required position on the clamp plate or an engagement lever which is electromagnetically driven when the pallet transfer is detected.
However, with the mechanism using a pin, when the pallet abuts against the stop for the temporary positioning, the stop prevents only the further forward movement of the transferred pallet. Therefore, it is necessary to continuously press the pallet against the stop and to hold the panel in the temporary position until the positioning pin is inserted into the pallet. This means that a specific operation is required, in which the pallet is held urged against the stop.
Further, the mechanism using an electromagnetically driven engagement lever requires means for detecting the pallet transfer of a pallet plate and an actuator for driving the engagement lever, leading to a bulky structure. Further, for lowering the clamp plate, a hydraulic or pneumatic actuator or a lead screw mechanism is used.
The inventors have studied a clamping device which simply clamps and releases an article having a predetermined thickness only by turning a clamp operating handle through an angle less than one revolution. The clamping device comprises a shaft fixed by the clamp operating handle and rotatably supported on a cramp base, and an eccentric integral with the shaft and fitted in one end of a crank lever having the other end integrally connected to the crank base movable vertically with respect to the clamp base.
In the clamping device of this type, the clamp plate is vertically moved in accordance with the eccentricity of the eccentric. With this type of the clamp device, however, the clamp operating handle must be turned until the clamp plate is lifted to its lowermost position in order to clamp the article completely. Therefore, the distance between the clamp base and the clamp plate at its lowermost position must be set extremely accurately.
In order to set the distance to such a high accuracy, however, various components that determine the distance must be manufactured at a very high accuracy. Moreover, even if the machining accuracy of individual components is improved, cumulative assembling errors would appear. Therefore, the clamping device of this type is inevitably low in the efficiency of manufacture and yield.
When a pallet is temporarily positioned in contact with the stop, a specific operation of pressing the pallet against the stop is required to maintain the pallet in the temporarily positioned state. Further, a positioning pin engaged with the temporarily positioned pallet projects upright from the clamp base. In order to project the positioning pin from the clamp plate to an extent necessary for the engagement of the pallet, the clamp plate must be lowered by a comparatively large distance equal to the extent of projection. Therefore, the maximum height level of the work to be machined, which is determined by the height position of the machining head of the machine tool, is reduced by an amount corresponding to the descent of the clamp plate, thus lowering the machining capacity of the machind tool.